In 3GPP (3rd Generation Partnership Project) that standardizes mobile communication systems, 3GPP-LTE/SAE (Long Term Evolution/System Architecture Evolution) is being standardized as the next mobile communication system.
3GPP-LTE/SAE standards have proposed radio access technologies (hereinafter “RATs”) each standardized in 3GPP in the past. 3GPP-LTE/SAE standards disclose the steps of inter-RAT handover, which is used by switching between the communication interface for the mobile communication system for the second generation (GS: Global System for Mobile Communication or GPRS: General Packet Radio Service) or the third generation (UMTS: Universal Mobile Telecommunication System) and the communication interface for 3GPP LTE.
Inter-RAT handover is so called “vertical handover” and refers to handover between different systems (or networks).
With inter-RAT handover in systems defined in 3GPP, whether to execute handover is decided on the mobile communication network side.
When the conditions for executing inter-RAT handover are fulfilled, a data transfer communication path from an access network that the communication interface which a mobile communication apparatus is currently connected with, utilizes to the access network that the communication interface utilizes after the switch, is secured in the access network. Then, a handover execution command message (a handover command) is transmitted to the mobile communication apparatus and makes the mobile communication apparatus execute inter-RAT handover processing.
Upon receiving the handover command, the mobile communication apparatus disconnects the connection with the communication interface currently in use, performs connection processing of the communication interface designated in the handover command, and switches the communication interface to utilize.
Communication data delivered to the access network before the switch during inter-RAT handover is transferred to the access network to be used by the communication interface after the switch through the data transfer communication path secured as described above, or is subject to bi-cast (e.g. Non-Patent Document 1).
Further, the TCP protocol is mainly used as a data communication method on the Internet or in mobile communication systems. With the TCP protocol, the actual amount of consecutive data transmission is determined from an advertised window and a congestion window (e.g. Non-Patent Document 2). The advertised window shows the amount of data which a TCP data segment receiving apparatus is then able to receive, to the access network. Also, the congestion window shows the amount of consecutive data which a TCP data transmitting apparatus estimates not to cause network congestion even if the TCP data transmitting apparatus continuously performs transmission, to the access network.
A mobile communication apparatus having a communication interface for a mobile communication network and a communication interface for a wireless LAN is a conventional mobile communication apparatus having different communication interfaces. A window control technique is known as a technique of smoothly performing communication upon switching a communication interface in this mobile communication apparatus. With this window control technique, the initial TCP window size for each communication interface is set in advance. Then, upon switching from a low speed communication interface to a high speed communication interface, a mobile communication apparatus transmits a TCP-ACK packet set in the window size for the switch destination communication interface (hereinafter “ACK packet”) before the communication interface switches.
With this technique, a mobile communication apparatus is configured to start handover processing. Then, with this technique, the link is switched according to the received signal strength from a base station, the IP layer is switched by mobile IP protocol processing, and the TCP window is set.
Accordingly, data is transferred after the IP layer has been switched, and therefore, to transmit the advertised window size to the communicating party after handover starts and before handover ends, it is necessary to access the communication interface after the switch meanwhile perform soft handover to maintain the connection with the communication interface before the switch (e.g. Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-153321    Non-Patent Document 1: 3GPP TS23.401 V1.2.1 [online] searched on Jan. 23, 2008, the Internet <URL:http://www.3gpp.org/ftp/Specs/html-info/23401.htm>    Non-Patent Document 2: RFC 2581 TCP [online] searched on Jan. 23, 2008, the Internet <URL:http://www.ietf.org/rfc/rfc2581.txt?number=2581>